Various exhaust-gas purification systems are known. These include systems which reduce the nitrogen oxide content in exhaust gases using externally infed reducing agents. The reducing agent is generally injected into the exhaust-gas flow by means of an injection device. A so-called SCR catalytic converter arranged downstream of the injection device then effects the actual conversion. SCR (selective catalytic reduction) refers to the technique of the selective catalytic reduction of nitrogen oxides in exhaust gases of combustion plants, refuse combustion plants, gas turbines, industrial plants and engines. The chemical reaction in the SCR catalytic converter is selective, that is to say preferentially the nitrogen oxides (NO, NO2) are reduced whereas undesired secondary reactions (such as for example the oxidation of sulfur dioxide to form sulfur trioxide) are substantially suppressed. SCR catalytic converters are often used in combination with soot particle filters and oxidation catalytic converters.
A reducing agent is required for the abovementioned reduction reaction, with ammonia (NH3) typically being used as reducing agent. Here, the ammonia required is generally used not directly, that is to say in pure form, but rather is used in the form of a 32.5% aqueous urea solution, referred to uniformly in the industry as AdBlue®. The composition is regulated in DIN 70070. The reason why the ammonia required is not carried on board in pure form is the fact that this substance is hazardous. Ammonia has a caustic effect on skin and mucous membranes (in particular on the eyes), and furthermore it forms an explosive mixture in air.
When the abovementioned urea solution is injected into the hot exhaust-gas flow, ammonia and carbon dioxide are formed from it through a decomposition reaction. The ammonia generated in this way is then available in the SCR catalytic converter arranged downstream. During the conversion of ammonia with the nitrogen oxides in the exhaust gas, a comproportionation reaction takes place, with water (H2O) and nitrogen (N2) being formed. With SCR catalytic converters, a distinction is typically made between two different types of catalytic converters. One type is composed substantially of titanium dioxide, vanadium pentoxide and tungsten oxide. The other type uses zeolites.
The amount of urea injected is dependent on the nitrogen oxide emissions of the engine and therefore on the present rotational speed and the torque of the engine. The consumption of urea-water solution amounts to approximately 2 to 8% of the diesel fuel used, depending on the untreated emissions of the engine. It is therefore necessary for a corresponding tank volume to be provided on board, which is in part perceived to be disadvantageous. In particular, this opposes the use in diesel-operated passenger motor vehicles, because an additional tank is provided.
Nitrogen oxides are removed from the exhaust gas to a great extent by means of selective catalytic reduction. In contrast to diesel particle filters (DPF), which are likewise known, or LNT (lean NOx trap) catalytic converters, there is no excess fuel consumption for the reduction of pollutants, because in contrast to the abovementioned catalytic converters, an SCR catalytic converter does not utilize any temporary deviation from optimum combustion conditions during operation.
When using SCR technology in utility vehicles, for example, the ammonia, in the form of AdBlue®, required for operation gives rise to further requirements. Owing to its particular properties, it may be carried on board as a further operating medium in a high-grade steel or plastic tank, and continuously injected into the exhaust-gas flow. As a result, aside from the SCR catalytic converter and the injection system, there is a need for a second, usually smaller tank aside from the diesel tank.
Furthermore, it may be noted that, during operation, AdBlue® may be injected in a variable fashion. Hitherto, the AdBlue® has been adapted to the NOx in the exhaust-gas mass flow by means of a so-called feed ratio. Here, if too much urea is dosed in, the ammonia formed from this can no longer react with NOx. In the event of such an incorrect dosing, ammonia can pass into the environment. Since ammonia is perceptible even in very small concentrations, this leads to an unpleasant smell.
It may be noted here that SCR catalytic converters are capable of adsorbing ammonia. As is the case in most adsorption and desorption processes, the adsorption of ammonia on the surface of the SCR catalytic converter material is also highly temperature-dependent. Accordingly, it is possible at relatively low exhaust-gas temperatures for relatively large quantities of ammonia to be adsorbed, which ammonia is desorbed again at higher temperatures if it has not in the meantime been consumed by means of the above-explained comproportionation reaction with nitrogen oxides.
The constantly changing operating conditions of an internal combustion engine have the effect that the exhaust-gas temperatures also constantly change during operation, and therefore the temperature of the SCR catalytic converter and, in turn, the ammonia adsorption capability thereof also constantly change. It is consequently difficult to dose in a quantity of reducing agent adequate to provide the required quantity of ammonia for all operating states of the internal combustion engine.
For example, the amount of reducing agent (e.g., ammonia) dosed to the SCR catalyst typically is based on a difference between a desired ammonia storage level and an estimated ammonia storage level, where the estimated amount of ammonia storage is determined based on catalyst temperature, catalyst age, ammonia concentration at the SCR inlet, etc. Such a mechanism for dosing the ammonia does not provide for quick response to rapidly changing exhaust conditions. As an example, if an accelerator tip-in occurs leading to an increase in exhaust gas temperature, by the time the temperature sensor has responded to the change in catalyst temperature, a significant amount of ammonia may be desorbed from the SCR, leading to ammonia slip.
The inventors herein have recognized the issues with the above approaches and provide a method to at least partly address them. In one example, a method for controlling an injection device for feeding an ammonia-releasing reducing agent into an exhaust-gas purification system of an internal combustion engine to reduce nitrogen oxide emissions, the exhaust-gas purification system comprising an SCR catalytic converter with n cells which are arranged in series in an exhaust-gas throughflow direction and in which ammonia is stored comprises determining a desired degree of ammonia loading at an exhaust-gas inlet temperature T0 and determining an ammonia partial pressure in exhaust gas directly upstream of the SCR catalytic converter; determining an actual degree of ammonia loading of the SCR catalytic converter by adding each individual degree of ammonia loading of each cell i of all of the n cells of the SCR catalytic converter, the individual degree of ammonia loading of each cell i determined as a function of a temperature Ti of the cell i and of an ammonia outlet partial pressure of a neighboring cell i−1 which is directly adjacent in an upstream direction; and determining a difference between the actual degree of ammonia loading and the desired degree of ammonia loading. If the actual degree of ammonia loading is less than the desired degree of ammonia loading, the amount of injected ammonia is increased, and if the actual degree of ammonia loading exceeds the desired degree of ammonia loading, the amount of injected ammonia is reduced.
In this way, the dynamics of ammonia storage/release in the SCR catalyst may be accounted for in order to maximize the NOx conversion and minimize the amount of ammonia slip using a one-dimensional model of the SCR catalyst. By dividing the SCR catalyst into a discrete number of cells, the ammonia loading at each cell may be determined rather than determining a single value for ammonia loading of the entire catalyst. Thus, the SCR catalyst model may account for temperature variations across the catalyst, such as those that may occur due to sudden acceleration or deceleration events.
The above advantages and other advantages, and features of the present description will be readily apparent from the following Detailed Description when taken alone or in connection with the accompanying drawings.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.